Face has different shading that rest of object
Go into edit mode, select the face(s) that are shaded funny.  Click on the shading/UVs tab on the tools menu, if you don't see the toolshelf, press T.  After clicking on the shading/UVs tab in the toolshelf [NB: "Shading/UV" was in a panel on the main toolshelf through version 2.69; the buttons were moved to a tab starting with version 2.7] click "flip direction" and it will flip direction of the normals of the face(s). Alternatively you could select the whole mesh and click recalculate, however that doesn't always work.